Filament structure



sept- 12 1950 I A. K. WING, JR 2,522,419

`FILAMENT STRUCTURE Filed Oct. 191946 Z9 /f z@ Z7 zi 25 Z7 lyj. if Z Y j +751 7 iM nventor Aww/f Miam 8u W Gttorneg heated cathodes.

Patented Sept. 12, 195@ f1 Arthur Kyle Wing, Jr., Nutley, N. il., assigner to i Radio Corporation. of America, a corporation of l' y Delaware Application October 19, 194:6-,V Serial No. 704,355

My invention relates `to a filamentary structure for thermionic tubesy and more particularly to a larnentary structure which is useful as a :lilamentary cathode, particularly in. high power thermionic tubes, and as a heater for indirectly .Filamentary cathodes which cathode emission for high power thermionic tubes at a filament temperature low enough to give Satisfactory life must have .ample filament sur-V face area. The conventional double helical or.

nected helices or helical coils of the same pitchr with the turns of one coil interleaved with the turns `of the other coil. As the adjacent turns carry current in opposite directions, the magnetic forcesbetween theadjacent turns or strands tend to maintainthe spacing between the turns, hence the structure is inherently stable. When the. two helical coils are operated in parallel with the current flowing in the same direction in both coils, the structure `is unstable,ras any deviation in spacing between-adjacent turns results in a cumulative` distortion which pulls the turns together. The surface area of a double helical filamentary cathode can be increased by decreasing the pitch so that more turns are wound in the'. available space, but this expedient reduces the mechanical stiifness of the cathode. A double.

helical filament could .be extended. by adding more turns of the same pitch, preferably with` a simultaneous increase in pitch to counteract.

the decrease in stiifness resulting from the longer helixwith the disadvantage that the physical length of the cathode is increased with correspondingly higher interelectrode capacitances, which is objectionable in tubes intended for operation at high frequency.v

The principal object of. my invention is to provide a stable lamentary structure which has considerably greater stability. and surface area than a .conventionaldouble helical reverse coiled filament occupying thesame space, and which is suitable for high power thermionic. tubes and particularly for tubes operated at high frequencies, Vsuch as 300 megacycles. Another Objectis to provide a lamentary structure which is useful as a heater for indirectly heated cathodes on account of its inherent stability and almost complete freedom from hum due to the magnetic ilelds set up when the filament is energized with alternating current..n

have adequate s claims. (c1. 25o-21.5)

In accordancewith my inventionl increase the 5 emissive area the cathode by using a pluralityv of vpairs of helices or helical :coils connected in such a way that adjacent turns carry current in opposite directions, thereby obtaining a lament surface greater than that of a conventional doublel helicaliilamentand an increasing inherent stan'J bility. at least equal to that of the double helical fl1ament.'. The lamentary structure preferably! comprises an evennumber of helices or helical. coils threaded one within the other with each. turn of each helix juxtaposed to the turns of: the helix next to it, so that each turn of av helix` lies alongside a neighboring turn. All of these helices arevjoned and electrically connected atf one end of the structure at a common point whichi is electrically the mid-point of the filament cath- 1 ode. The otheror free ends of the helices are-so connected to the current supply terminals -thatI` current flows in 'one direction through one helixfl and .inthe opposite direction through the helix. In every1? turn'of veach helix the direction` of current flowis' oppositelto that ofthe current now in the next orA neighboring turn of the adjacent helix` A`Thefree -ends ofthe helices opposite to the ends1 nextin ordergaroundv the structure.

which are joined at la. common point are arranged irl-"a Irow or array, preferably on the circumference of acircle, and-in the same order in which' thev helices Vare positioned valong their commonA longitudinal axis. By connecting one current supply' terminal to the freeend of every other;` or alternate helix in order around the array, and connecting the other current supply terminal to everyother or alternate free end inthe array,v

the current `flows through the helices in such rent in opposite directions.

Figure 1 alonglthe line 2 2.

@In the particular form of filamentary struc? ture `which. I have shownmerely for illustration of one'embodiinent-of my invention a glass stern I0,'Ipreferably of the discor cup-type, carries 'a' mount which has two rigid terminals H and i2 sealed into andv extending through the ste1n.- These terminals supply current to the filament' and also support the mount on the stem. -1The mount" comprises an insulating disc I3v4 which carries la metal iilamentsupport rod "I4 away that all of the adjacent turns carry cur!y at its center and near its rim four inner leads, I5, I6, I'I and I8 positioned at the corners of a square of which the support rod is the center. The two inner leads I5 and I8 extend through the disc I3, and their lower ends are electrically connected to the two terminals II and I2 respectively by sheet metal connectors I9, which are welded to the inner ends of the terminals and to the lower ends of the leads I5 and I8 and rigidly connect the terminals to the leads and also support the disc I3. The inner lead I5 is con; nected to the diametrically opposite inner lead II by a jumper 20, which at the middle is e'nlarged to encircle without touching the support rod, and inner lead I6 is similarly connected to? inner lead I8. The inner leads and the center support rod are preferably steadied b y an ine sulatingrspacer 2l. A cup-shaped metal shield 22 mounted on and electrically connected to the center support rod and having in the bottom four holes through which the inner leads extend without touching the shield surrounds the inner` leads near their upper ends. Electrical connection to the midpoint of the lamentary cathode may be made by a lead-in wire 23 sealed into the stemv and connected to the center support rod I4.

In accordance with my invention the lamen'- tary structure of the mount comprises four sub-v stantially identical helices or helical coils 25, 26, 2I and 28, all of the same diameter, length and pitch mounted coaxially and threaded together screw fashion and with their upper ends remote' from the stem all joined and electrically con-'-V nected to one another, for example by arc weld-f ing at a common point 29 which, in the embodiment shown, is also the upper end of the filament support rod I4. The other, or free' ends of the four helices are arranged in a row `or array about the center support rod kand each end is joined to an inner lead. The terminal II is connected to the free ends of helices 25 and 2'I whichr are connected in parallel to the common point 29, and the terminal I2 is connected tothe free ends of the helices 28 and 28 which are also connected in parallel to the `common point 29. Assuming for convenience that terminal II is positive, and that current flows from the positive terminal I I to the negative terminal I2, the. current. will flow through all four helices in the directions indicated by the arrows and in opposite directions in each turn of the adjacent helices. l For example, the current passes through helix 25 from the inner lead I'I to the common point 2 8 and thence through the next helix 26 to the inner lead I5, and so on in order around the array. As the helices are substantially identical, the same amount of current ilows through each helix. I have found thatin a lamentary structure made as shown in thezd'rawing with four helices and even with a pitch increased over whatA would ordinarily be used with the conventional double helix the filament surface available in the same overall physical space is about 60% more than with the conventional double helix. The increased pitch of each individual helix has resulted in a iilament assembly having markedly better stability than a conventional double helix filament. It is obvious that more than theA four strands shown in the drawing can be used, the only limitation on the number of strands being reasonable clearance between helices and the physical arrangement of the legs and the inner leads. A helical filament structure of large diam--A eter can be designed in accordance withmy'invention by using a steep pitch 'for each Vof :a

large number of strands, thus obtaining a stability not obtainable with the conventional double helix.

What I claim as new is:

l. A nlament structure for thermionic tubes comprising four substantially identical coaxial helices threaded one within the other with their turns juxtaposed and each having a turn lying substantially alongside a turn of a neighboring helix, a support rod extending through said helices, one end of said support rod being connected to adjacent ends to said helices, four inner leads of relatively large cross-section connected to the' other endsl of said helices for effectively "siip'portiiig` said' helices and for providing good electrical connection thereto, alternate ones of saidinner leads-being connected electrically for providing current vilow in the same direction in alternate ones of said helices, and two lead-ins connected to said inner leads whereby induction eiects on each helix is neutralized and the current in the several helices is maintained substana tially the same.

2. A filament structure for' thermionic tubes comprising va plurality oi-pairs of coaxial coils of the same pitch and size threaded into one another with their same ends adjoining and electrically connected to a' power supply of the same pin-- tential` the other ends of -said coils being are ranged -in a row-in the same order as said coils are positioned along their longitudinal axis,- a'

current supply conductor connected to said other ends of alternate coils, and a second current supply conductor connected to said other endsof the remainder of said'coils.

3. A filament structure for thermionic tubesy 4. A lament structure for thermionic tubes comprisingA a plurality of pairs of parallel con-g nected coaxial lament coils threaded into one another with juxtaposed turns equally spaced from one another, said coils electrically connect'my ed by a single centrally locatedsupport at one Iend and having their opposite ends .arranged side-bye side in an array in the same order as said coils are distributed along their longitudinall 'ax-is, apair -of current supplyterminals, and means for electrically connecting alternate filament ends of said array to one terminal andthe otherfllament ends to the other terminal, whereby said coils of each group vare connected: in parallelfai-1dl current lows in one direction in Ione coil and'in the oppo" site direction in the nextcoilifn order around the array. f

5; AA lila-ment structure for thermionic tubescomprising a plurality' of pairs of substantially identical coaxial helical coils threadedone witlfiin the'other, each of said'vcoils comprising ai sep# arate element, the-ends of said coils at one endet said structure being relatively closely spaced fromI each other, a rodv extending through said Acoils longitudinally thereof, one end of said'rod-beihg connected to said ends ofv said coils, the other ends ofl said coils being farther spaced from eachother than the first-mentioned ends, inner leads of relatively large cross-section connected lto saidy other ends of said coils, said inner leads being disposed in a planar array wherein alternate; vones of said leads are connected to alternate onesjof said coils longitudinally of said structure'gtland two other electrical conductors of relativelylarge cross-sectionconnected to said alternatejileads whereby a rugged structure is provided andfffreedom from induction effects in the coils is assured.

6. A filament structure for thermionic'gtubes comprising four identical coaxial helicaljcoils threaded into each other with each turnf two intermediate coils between the turns; neighboring coils andthe turns of all saiH L l* juxtaposed and equally spaced, a single conductive support joined to the same end of sai" whereby the potential applied to said suppv be the same for each of said coils, and tw rent supply conductors at the other ends;` said coils, said other end s of alternate coils being connected to one of said conductors an "id other ends of the remainder f said coil nected to the other conductor to cause t rection of current flow in each turn of each coil to be opposite to the direction of current ow in the turn next to it. l

7. A filament structure for thermionicgftubes having a plurality of pairs of substantiallyfsimilar axially disposed lbut angularly displacedcoils whereby the coils are threaded one withinthe other and halving a set of adjacent ends disposed in a substantially circular array, inner le'ds of relatively large cross-section disposed in parallel relation andgconnected to said coils attheirjsaid adjacent ends,;`a rod support extending through said structure,the other set of adjacent ends of said coils being relatively closely spaced from each other and connected to one end of said support rod, a lead-in for said structure, said lead-in being connected to alternate ones of said inner leads, whereby alternate ones of said coils receive current in the same direction.' and anotherleadin connected to the other of said inner leads whereby the other of said coils receive current in a direction opposite to said first-named direction whereby induction eiects in the coils are neutralized.

8. A mount for an electron discharge device including a fllamentary cathode andsupport therefor, said cathode comprising a plurality of coaxial coils threaded into one another, said support comprising means at one end of said mount for supporting said coils comprising a conducting rod supported at the other end of said mount and extending longitudinally through saidjcoils, one set of adjacent ends of said coils beingfgconnected to one end of said conducting rod, a plurality oi' additional conducting rods connectedfto the opposite adjacent ends of said coils, an' insulating support for said conducting rod and` s'jaid additiona1 conducting rods, and tWo lead-ins for said cathode, said lead-ins supporting said insulating 81113D011- ARTHUR KYLE WING, Jn. v

REFERENCES CITED The following referencesv are of record in the ille of this patent:

UNITED STATES PATENTS Glalliier Mar. 1, 1949 

